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Selected Balance Sheet Data Balance Sheet Related Disclosures, Inventories (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Public Utilities, Inventory [Line Items]    
Inventories $ 33,401 $ 40,433
Materials and supplies    
Public Utilities, Inventory [Line Items]    
Inventories 25,380 26,218
Fuel    
Public Utilities, Inventory [Line Items]    
Inventories $ 8,021 $ 14,215